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They said the Markham Ice Shelf, one of just five remaining ice shelves in the Canadian Arctic, split away from Ellesmere Island in early August. They also said two large chunks totaling 47 square miles had broken off the nearby Serson Ice Shelf, reducing it in size by 60%. “The changes … were massive and disturbing,” said Warwick Vincent, director of the Centre for Northern Studies at Laval University in Quebec on Tuesday.

Temperatures in large parts of the Arctic have risen far faster than the global average in recent decades, a development that experts say is linked to global warming.

“These substantial calving events underscore the rapidity of changes taking place in the Arctic,” said Derek Mueller, an Arctic ice shelf specialist at Trent University in Ontario. “These changes are irreversible under the present climate and indicate that the environmental conditions that have kept these ice shelves in balance for thousands of years are no longer present,” he said in an emailed statement from the research team sent late on Tuesday.

Mueller said the total amount of ice lost from the shelves along Ellesmere Island this summer totaled 83 square miles — more than three times the area of Manhattan island.

The figure is more than 10 times the amount of ice shelf cover that scientists estimated on July 30 would vanish from around the island this summer. “Reduced sea ice conditions and unusually high air temperatures have facilitated the ice shelf losses,” said Luke Copland of the University of Ottawa.

“Extensive new cracks across remaining parts of the largest remaining ice shelf, the Ward Hunt, mean that it will continue to disintegrate in the coming years,” he said.